Images in CMOS image sensors are captured using a rolling shutter methodology. In rolling shutter operation, the image is captured on a row-by-row basis, where for a given row the image is captured in its light conversion unit, transferred to a floating diffusion node, and then read out of the pixels to column sample circuits before moving on to the next row. This repeats until the all the rows are captured and read out sequentially or in some other sequence. In this methodology, each row captured actually represents the subject (image) at a different time. Thus, for highly dynamic subjects (e.g., such as objects moving at a high rate of speed) the rolling shutter methodology can create image artifacts.
A global shutter methodology is used to solve this image artifact issue of capturing high speed objects. For a global shutter operation, the image is captured for the whole frame in the light conversion units of the pixels at the exact same time for all the rows and columns. The signal is then transferred to the floating diffusion (FD) node where it waits until it is read out of the imager array on a row-by-row basis. The global shutter method solves the problem with image capture of high speed subjects, but introduces a concern with the global shutter efficiency of the pixel.
In the rolling shutter method, the image signal is held onto the floating diffusion for a significantly shorter time than the actual time of exposure in the light conversion unit, e.g. photodiode. Thus, the contribution of the generation rate of the floating diffusion is orders of magnitude smaller than the generation rate during the integration time in the light conversion structure, e.g. photodiode. This hold time on the floating diffusion is constant for all pixels in the imager array.
But for the global shutter method, the image signal is held onto the floating diffusion for varying amounts of time, with the first row only waiting for the time to read out a single row, while the last row signal waits on the floating diffusion for the full frame read-out time. Thus, any generations or leakage occurring on the floating diffusion node can have a significant impact to the signal being read out of the imager. The global shutter efficiency of the pixel is determined by the ratio signal read out of the pixel versus the initial signal captured by the pixel. Ideally the signal read out would be exactly the signal captured.
In order to improve on the global shutter efficiency it is necessary to reduce the amount of change to the signal being held on the floating diffusion. Various phenomenon can impact this efficiency, such as photon generated carriers being collected in the floating diffusion, floating diffusion junction leakage to substrate, and floating diffusion leakage through the reset gate (RG) to Vdd node.
Even with all the available process variations and additions to minimize the leakage on the floating diffusion, there may always be some level of photon induced leakage occurring on the floating diffusion node and therefore global shutter efficiency may never be 100%. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.